FIX: ISHFT and ISHL Always Assume 4-Byte Integers

Last reviewed: September 18, 1997
Article ID: Q111234
1.00 1.0a | 1.00
MS-DOS    | WINDOWS NT
kbtool kbfixlist

The information in this article applies to:

  • Microsoft FORTRAN PowerStation for MS-DOS, version 1.0 and 1.0a
  • Microsoft FORTRAN PowerStation 32 for Windows NT, version 1.0

SYMPTOMS

The ISHL and ISHFT intrinsic functions may return incorrect results when shifting INTEGER*1 or INTEGER*2 variables. The incorrect results occur only when right-shifting negative values.

CAUSE

The compiler converts the value to be shifted to a 4-byte value. For 1- and 2-byte negative integers, the higher order bits are filled with 1s, which can be incorrectly shifted into the low order bytes.

RESOLUTION

Convert the bit pattern of the negative INTEGER*1 or INTEGER*2 value to an unsigned INTEGER*4 value. This is done by adding 256 to a negative INTEGER*1 value or by adding 65536 to a negative INTEGER*2 value. Then shift the resulting INTEGER*4 value.

STATUS

Microsoft has confirmed this to be a problem in FORTRAN PowerStation 32 for Windows NT version 1.0 and MS-DOS version 1.0 and 1.0a. This problem was fixed in FORTRAN PowerStation 32, version 4.0.

MORE INFORMATION

The following sample illustrates the problem:

Sample Code #1

c Compile options needed: none c

      integer*2 i2
      integer*1 i1
      i1 = -123
      i2 = i1
      write(*,'('' Before shift '',Z2,2x,z4)') i1, i2
      write(*,'('' After shift  '',Z2,2x,z4)') ishl(i1,-2), ishl(i2,-2)
      end

Output

Before shift 85 FF85

After shift   E1  FFE1

The following code illustrates the article's resolution:

Sample Code #2

c Compile options needed: none c

      integer*4 j,k
      integer*2 i2
      integer*1 i1
      i1 = -123
      i2 = i1
      j = 256 + i1
      k = 65536 + i2
      write(*,'('' Before shift '',Z2,2x,z4)') i1, i2
      write(*,'('' After shift  '',Z2,2x,z4)') ishl(j,-2), ishl(k,-2)
      end

Output

Before shift 85 FF85

After shift   21  3FE1


Additional reference words: 1.00
KBCategory: kbtool kbbuglist
KBSubcategory: FORTLngIss
Keywords : FORTLngIss kbbuglist kbtool
Version : 1.00 1.0a | 1.00
Platform : MS-DOS NT WINDOWS
Solution Type : kbfix


THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.

Last reviewed: September 18, 1997
© 1998 Microsoft Corporation. All rights reserved. Terms of Use.